This invention relates to a fuel cell system and more particularly to a system having a plurality of cells which consume an H2-rich gas to produce power.
Fuel cells have been used as a power source in many applications. For example, fuel cells have been proposed for use in electrical vehicular power plants to replace internal combustion engines. In proton exchange membrane (PEM) type fuel cells, hydrogen is supplied to the anode of the fuel cell and oxygen is supplied as the oxidant to the cathode. PEM fuel cells include a membrane electrode assembly (MEA) comprising a thin, proton transmissive, non-electrically conductive solid polymer electrolyte membrane having the anode catalyst on one of its faces and the cathode catalyst on the opposite face. The MEA is sandwiched between a pair of electrically conductive elements which (1) serve as current collectors for the anode and cathode, and (2) contain appropriate channels and/or openings therein for distributing the fuel cell""s gaseous reactants over the surfaces of the respective anode and cathode catalysts. The term fuel cell is typically used to refer to either a single cell or a plurality of cells (stack) depending on the context. A plurality of individual cells are commonly bundled together to form a fuel cell stack and are commonly arranged in series. Each cell within the stack comprises the membrane electrode assembly (MEA) described earlier, and each such MEA provides its increment of voltage. A group of adjacent cells within the stack is referred to as a cluster. Typical arrangements of multiple cells in a stack are described in U.S. Pat. No. 5,763,113, assigned to General Motors Corporation.
In PEM fuel cells, hydrogen (H2) is the anode reactant (i.e., fuel) and oxygen is the cathode reactant (i.e., oxidant). The oxygen can be either a pure form (O2), or air (a mixture of O2 and N2), though in this specification the term xe2x80x9cairxe2x80x9d is used to refer to both O2 and O2 in combination with other gases. The solid polymer electrolytes are typically made from ion exchange resins such as perfluoronated sulfonic acid. The anode/cathode typically comprises finely divided catalytic particles, which are often supported on carbon particles, and mixed with a proton conductive resin. The catalytic particles are typically costly precious metal particles. These membrane electrode assemblies are relatively expensive to manufacture and require certain conditions, including proper water management and humidification, and control of catalyst fouling constituents such as carbon monoxide (CO), for effective operation.
For vehicular applications, it is desirable to use a liquid fuel such as an alcohol (e.g., methanol or ethanol), or hydrocarbons (e.g., gasoline) as the source of hydrogen for the fuel cell. Such liquid fuels for the vehicle are easy to store onboard and there is a nationwide infrastructure for supplying liquid fuels. However, such fuels must be dissociated to release the hydrogen content thereof for fueling the fuel cell. The dissociation reaction is accomplished within a chemical fuel processor or reformer. The fuel processor contains one or more reactors wherein the fuel reacts with steam and sometimes air, to yield a reformate gas comprising primarily hydrogen and carbon dioxide. For example, in the steam methanol reformation process, methanol and water (as steam) are ideally reacted to generate hydrogen and carbon dioxide. In reality, carbon monoxide and water are also produced. In a gasoline reformation process, steam, air and gasoline are reacted in a fuel processor which contains two sections. One is primarily a partial oxidation reactor (POX) and the other is primarily a steam reformer (SR). The fuel processor produces hydrogen, carbon dioxide, carbon monoxide and water. Downstream reactors may include a water/gas shift (WGS) and preferential oxidizer (PROX) reactors. In the PROX, carbon dioxide (CO2) is produced from carbon monoxide (CO) using oxygen from air as an oxidant. Here, control of air feed is important to selectively oxidize CO to CO2.
Fuel cell systems which process a hydrocarbon fuel to produce a hydrogen-rich reformate for consumption by PEM fuel cells are known and are described in co-pending U.S. patent application Ser. Nos. 08/975,422 and 08/980,087, filed in November, 1997, and U.S. Ser. No. 09/187,125, filed in November, 1998, and each assigned to General Motors Corporation, assignee of the present invention; and in International Application Publication Number WO 98/08771, published Mar. 5, 1998. A typical PEM fuel cell and its membrane electrode assembly (MEA) are described in U.S. Pat. Nos. 5,272,017 and 5,316,871, issued respectively Dec. 21, 1993 and May 31, 1994, and assigned to General Motors Corporation.
Efficient operation of a fuel cell system depends on the ability to effectively control system shutdown, particularly in a rapid shutdown mode. One particularly important system component in this regard is the air compressor, which delivers air/O2 to the fuel cell system. Typically, the compressor operates at approximately 200xc2x0 C. outlet temperature. This typical outlet temperature is very close to that which would degrade the compressor, that is, a temperature of approximately 210xc2x0 C. A compressor overtemperature/overpressure condition can degrade the compressor, as well as sensitive downstream system components. However, while turning off the air compressor can alleviate the undesirable effects of an overtemperature/overpressure condition, the lack of air to the system can degrade other system components, including the fuel-cell, combustor, and reformer/fuel processor, all of which rely on airflow during shutdown. For instance, air flow to the combustor must be maintained during shutdown to prevent overheating as the combustor burns off residual gases. Accordingly, it is desirable to provide a method and apparatus by which a compressor overtemperature/overpressure condition can be alleviated during rapid shutdown without depriving other system components of necessary airflow.
In one aspect, the invention provides a venting methodology and system for relieving fuel cell system overpressure, particularly during rapid system shutdown, while maintaining airflow through the system. In a further aspect, there is provided a preferred valving and control arrangement for carrying out the inventive methodology.
In one arrangement there is provided a fuel cell system comprising, in fluid communication, an air compressor having an outlet for providing air to the system, a combustor operative to provide combustor exhaust to the fuel processor, and at least one valve for selectively venting combustor exhaust from the system when the fuel processor is reforming. The invention further provides selectively venting combustor exhaust via the at least one valve when the fuel processor is not reforming and when the air compressor is operating outside of one or more predetermined parameters.
According to one feature of this methodology, the operating condition of the fuel processor (i.e., whether it is operating to produce a reformate, such as H2 gas) is determined, the condition of at least one operating parameter of the air compressor is determined, and combustor exhaust is selectively vented via the at least one valve if the fuel processor is not reforming and the at least one operating parameter of the air compressor is determined to exceed one or more predetermined values.
According to another feature of the invention, the step of determining the condition of at least one operating parameter of the air compressor comprises determining temperature and/or pressure conditions proximate the compressor outlet. According to this feature, combustor exhaust is selectively vented if the air compressor is operating above predetermined temperature and/or pressure values.
According to another inventive feature, the step of selectively venting combustor exhaust further comprises opening the at least one valve for a predetermined period of time. This predetermined period of time is, according to one feature of the invention, approximately one minute.
The invention methodology is carried out by at least one vent valve provided in the system flow path, for instance between the combustor and reformer, to vent combustor exhaust in response to a condition of fuel processor overheating. The control logic as adapted to this system determines one or more parameters reflecting the operating conditions of the reformer and the air compressor, and directs the at least one vent valve to open, even in the event that the reformer is not reforming, when the air compressor is determined to be operating outside of one or more predetermined parameters, such as an overtemperature/overpressure condition, all according to the inventive methodology. Control of the vent valving can be through a dedicated controller comprising any suitable microprocessor, microcontroller, computer, etc. which has a central processing unit capable of executing a control program and data stored in memory, or other suitable means.